Prior to the present invention, as shown by Chen et al, U.S. Pat. No. 4,362,809, for Multilayer Photoresist Process Utilizing an Absorbant Dye, polymethylmethacrylate (PMMA) photoresist compositions containing coumarin as an absorbant dye, were used as a photoresist for etching semiconductor devices. Although the coumarin containing PMMA compositions of Chen et al were found useful in the practice of multilayer photoresist methods using an upper portable conformable mask (PCM), coumarin was found to interfere with the delineation of the lower PMMA resist layer during its subsequent UV exposure in the range of about 220-250 nm. As a result, an increase in the duration of ultraviolet light exposure time of the PMMA layer by a factor of at least two was found necessary to overcome the coumarin absorption in the region of 220-250 nm.
It would be desirable, therefore, to provide a PMMA photoresist composition having an absorbant dye which performed satisfactorily in a multilayer photoresist process using a portable conformable mask. In order to perform satisfactorily, the dye would have to be soluble in the PMMA and sufficiently non-volatile so that it could survive the initial spin and baking steps during the application of the PMMA on the substrate surface. The dye also would have to absorb at about 436 nm during the initial exposure of the upper positive resist layer to effect the formation of the PCM, which would substantially minimize unwanted reflections from the PMMA resist-substrate interface during such UV exposure. An additional criterion of the absorbant dye is that the dye would have to be substantially non-absorbing in the range of about 220-250 nm which would permit the delineation of the PMMA layer by a deep UV blanket through the PCM.
The present invention is based on the discovery that the certain derivatives of cinnamic acid having the formula ##STR1## where R and R.sup.1 are selected from hydrogen or C.sub.(1-8) alkyl, Q is --O-- or --N--, X is selected from halogen and nitrile, R.sup.2 is a monovalent group selected from ##STR2## polyalkylene glycol, polyalkylene amide and polyalkylene ester and R.sup.3 is a C.sub.(1-8) alkylene radical or C.sub.(6-13) aryl radical, have been found to be absorbant at about 400-465 nm, substantially transparent in the range of about 220-250 nm, soluble in an organic solvent solution of PMMA and in PMMA after spin casting and baking. There is shown in the figure, the extinction coefficients of the 1,3-propanediol diester of methyl(p-diethylamino-.alpha.-cyano)cinnamic acid and coumarin. These spectra were measured with a Perkin Elmer 330 UV Spectrophotometer in an ethanol solution.
The cinnamic acid derivatives of formula (I) which preferably can have a molecular weight in the range of about 350 to about 1000 are also substantially non-volatile at temperatures in the range of about 100.degree. C. to 200.degree. C. As a result, spin castable PMMA compositions are provided which are capable of absorbing in the 436 nm region while substantially transparent in the 220-250 nm region. As a result, these photoresists can be advantageously utilized in multilayer photoresists processes without the disadvantages of the prior art requiring excessive exposure time due to a high level of absorbance in the 220-250 nm region.